Responses of Halobacterium halobium cells to chemical stimuli have been shown by a capillary technique. Cells were attacted by D-glucose and several amino acids and repelled by phenol. Certain chemicals, such as acetate, benzoate, indole, and NiSO4, that are known to act as repellents of Escherichia coli cells served as attractants for Halobacterium. In the presence of ethionine, sensitivity to attractants was reduced. Arsenate prevented the attraction by glucose without lowering the cellular adenosine 5'-triphosphate level. The ability for chemo-accumulation toward glucose and histidine was interfered with by the formation of photosensory systems. Light-induced motor responses and chemosensory behavior toward glucose and histidine became detectable in the late stationary growth phase only. The behavior toward acetate and indole was not connected to photobehavior in that way: both substances acted as attractants already in the late log phase. Inhibition of bacteriorhodopsin synthesis by L-nicotine allowed chemo-accumulation toward glucose and histidine already in the late logarithmic phase.
In Halobacterium halobium, nicotine is known to block the synthesis of retinal. Cells grown in the presence of nicotine do not show any photophobic response. Addition of retinal1 or retinal2 restored the photophobic responses to light-increase in the UV and to light-decrease in the green-yellow part of the spectrum. The action spectra of the two retinal2-photosystems were red-shifted by 15--20 nm, compared with the corresponding retinal1 systems. We conclude that each of the two photosystems PS 370 and PS 565, has its own photosensory pigment with retinal as the chromophoric group.
Stimulation of Halobacterium halobium through its sensory photosystems, PS 370 and PS 565, leads either to a prolonged or to a shortened interval between two reversals of the swimming direction of the cell, the attractant or repellent response. Stimuli are integrated to yield the same response regardless through which photosystem they are given. Simultaneously elicited attractant and repellent signals cancel each other at any time during a reversal interval, even in the period of refractoriness shortly after a reversal, when the cell is insensitive to repellent stimuli. Successively applied stimuli are less completely integrated. The net response depends on the moment of stimulation during the interval, on the sequence of stimuli, and on the delay between them. Integration of successively applied effective stimuli (after refractoriness) is to a great extent explained in terms of a cellular oscillator (A. Schimz and E. Hildebrand, Nature [London] 317:641443, 1985) which is changed in opposite directions by attractant and repellent signals. Some conclusions on the shape of the oscillator after its disturbance by a stimulus can be made. Integration of signals during refractoriness leads us to postulate an additional step before the oscillator in the sensory pathway. Cancelling of simultaneous opposite signals is thought to proceed at this integrator. It also takes part in the integration of successively evoked signals. At this step signals rapidly decline within 10 ms, and the total life time (at least of repellent signals) does not exceed 1.2 s.Halobacterium halobium has polar flagella and can swim in either direction along its long axis. Owing to a change of flagellar rotation from clockwise to counterclockwise or vice versa (1), the cells reverse their swimming direction about every 10 s. Successive reversal intervals are, on average, equally long, i.e., the cells have no preferred swimming direction (6, 9).The organism has two retinal-dependent sensory photosystems of different spectral sensitivity, PS 370 and PS 565, through which its behavior can be controlled (2,3,12). It is still an open question whether two distinct proteins (7) or two states of a single molecule in a photochemical cycle (14) act as the receptors. Other retinal pigments and carotenoids contribute to photosensing (2,11,16). Light stimuli alter the interval between two reversals of the swimming direction. A light increase in the yellow-green range delays the next reversal and is therefore regarded as an attractant stimulus, while a light increase in the blue-UV range advances the next reversal and is regarded as a repellent stimulus. A light decrease in the yellow-green range, on the other hand, i.e., removal of attractant light, acts as a repellent stimulus, while a light decrease in the blue-UV range, i.e., removal of repellent light, acts as an attractant stimulus (3, 4, 15). Consequently, we define a stimulus-induced increase of the interval length as an attractant response and the decrease as a repellent response.The frequency distri...
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